In many areas of oil exploration and development, communication between the surface and downhole is vital but difficult. This is true from drilling through to production and intervention in existing wells. The typical problem is effecting a channel of communication, by some method, down a long conduit filled with fluid. In most situations, the conduit (for example, the borehole) is considered the only practical physical route for information, as electromagnetic or elastic waves are strongly attenuated by passage through thick layers of rock. Conventional methods include pressure waves in the fluid (e.g. mud pulse telemetry) or the use of electrical cables, extending the length of the borehole. These conventional methods have disadvantages, which include cost, reliability, and low data rate.
Some ideas have been proposed around the idea of sending some object or element up or down the borehole. A raw piece of semiconductor memory onto which data is written by a downhole device has been disclosed. For example see, GB patent application Ser. No. 1 549 307. A more sophisticated and robust vessel containing memory has been disclosed by GB patent No. 2 352 041, and co-pending U.S. patent application Ser. No. 10/049,749 assigned to Schlurnberger Technology Corporation published as PCT application WO 01/04661. Alternatively, even more complex vessels containing a variety of sensors and data storage have been disclosed. For example, see GB Patent No. 2 352 042, and PCT Published Applications WO 99/66172 and WO 01/04660.
U.S. Pat. No. 6,443,228 discloses the use of flowable devices in wellbores to provide communication between surface and downhole instruments, among downhole devices, establish a communication network in the wellbore, act as sensor, and act as power transfer devices. In some embodiments, the upwards communication is proposed by writing information on the flowable devices downhole which are bound for the surface.
Co-owned U.S. Pat. No. 6,915,848 (incorporated herein by reference) discloses a well control system enabling the control of various downhole well control functions by instructions from the surface without necessitating the well or downhole tool conveyance mechanism being equipped with electrical power and control cables extending from the surface and without the use of complex and inherently unreliable mechanical shifting or push/pull techniques requiring downhole movement controlled remotely from the surface. The invention of this co-pending application makes use of downhole well control apparatus that is response to instructions from elements such as fluids or physical objects such as darts and balls that are embedded with tags for identification and for transmission of data or instructions. According to at least one disclosed embodiment, a downhole device may also write information to the element for return to the surface.
In these disclosed embodiments, where information is being sent from a downhole location to the surface, information is written to the device (or acquired by the device itself) downhole.